One of the best all-purpose backs in the NFL, if not the best, Westbrook has occasionally become an afterthought in the Eagles' offense this season, with production far below the career numbers he posted in 2007.

The offense has stuttered and stumbled at important junctures during the season, most recently during the late loss against Washington, in which Westbrook carried the ball just a dozen times.

Why does Westbrook's role disappear sometimes - to the consternation of fans, of some of his teammates, and, one would think, of Westbrook himself?

"I don't have the answer for that," he said. "I just don't have the answer."

Andy Reid has the answer, but his explanations tend to wander a twisting path through the thickets of the West Coast offense. There are things he does not share with the outside world - there's a shock - and some of the reasons for Westbrook's decline in production and importance are open to speculation.

The truth is elusive here, and the key to unlocking the potential of the Eagles' offense isn't as simple as simply giving Westbrook the ball more often. For one thing, the offense set a franchise record for points this season, so a few things must be going right. For another, there are factors that forced Reid to pass more often than even his pass-happy philosophy usually dictates.

Still, Westbrook led the league in 2007 with 2,104 yards from scrimmage (rushing plus receptions). This season, his 1,338 yards from scrimmage places him 12th in the league. He gained 1,333 yards on 278 rushes a year ago and 771 yards on 90 receptions. This season, he gained 936 yards on 233 rushes and 402 yards on 54 receptions.

Westbrook missed two games because of injury this season, but that doesn't account for all of the downturn, and certainly not for the per-rush average of 4.0 yards, which ties the lowest mark of his career. He also missed one full game in 2007 and then got just seven carries in a meaningless season-ending game.

No, a combination of things this season has marginalized Westbrook. In no particular order:

The middle of the offensive line, with the absence of all-pro guard Shawn Andrews, was thin and not overly talented, a situation exacerbated when Max Jean-Gilles broke his leg Nov. 27. Without the ability to reliably open holes up the gut, Reid became wary of going there at all.

Westbrook has been gimpy for much of the season. He missed the Sept. 28 game in Chicago with an ankle injury and the Oct. 12 game in San Francisco with bruised ribs. The ankle injury lingered through much of the middle of the season, and he battles chronically sore knees each week. There are weeks - and this has been one of them - when his ability to practice is limited.

Reid and offensive coordinator Marty Mornhinweg fell in love with the versatility and explosiveness of rookie receiver DeSean Jackson, whose 62 receptions made him the first wide receiver to lead the team in catches since Terrell Owens in 2004. Jackson's 14.7-yards-per-catch average - also the highest for the leading target since Owens - made that switch in philosophy seem like a good idea. Suddenly, some of the sets in which Westbrook was split wide disappeared from the offense, and his receptions were very nearly cut in half.

With Westbrook not used as often as a receiver, the mathematics of Reid's version of the West Coast offense worked against him. In the first half of games, Reid likes to throw around 60 to 70 percent of the time. In the second half, if the Eagles are ahead, things get more balanced. But if the lead is comfortable and the Eagles are trying to keep Westbrook from getting more banged up, those second-half carries might be spread to others. A good example was Sunday's laugher against the Cowboys, when Westbrook had 13 carries and Correll Buckhalter 10.

Westbrook has been around here long enough to understand all of that, and he's a lot more philosophic about it this season than he would have been last season, when he was playing for the contract upgrade that he eventually received.

"I think it's scheme a little bit, but probably the biggest part of it is that I've been banged up a little bit," Westbrook said, analyzing his diminished role. "When you add DeSean in there, he's catching a lot of balls as well."

In a way it has added up to more - those 416 points are hard to ignore - even though it has also meant less of Brian Westbrook. That doesn't figure to change on Sunday against the stout rushing defense of Minnesota, and, if the Eagles fail, someone will ask why Westbrook wasn't used more often.

It would be nice if it were that simple, but for the Eagles this season, nothing is that simple.

Contact columnist Bob Ford at 215-854-5842 or bford@phillynews.com. Read his recent work at http://go.philly.com/bobford